Rudder vibrations Help

Just took over a good old working boat. I have vibrations and noise from the rudder and is pretty much is in all the boat. Is a steel working boat. I can see how the bracket for the rudder is shaking but is only in between 1600 rpm and 2000 rpm 5,5 - 8,5 knots and at full speed is very little 2300 rpm 10 knots. I have check all bearings and mounting and is okey. The propeller is new balance and axel is checked.

Can it be the rudder design that is bad?
Any ideas is appreciated. See pictures on the rudder.

 

This is how the boat is looking

 

Welcome to the forums

Let us say the rudder design looks "sub-optimum"... Can you post a picture from dead astern? I would like to see if the rudder is offset in order to pull the propeller shaft. And it also looks like the rudder is "wedge" shaped.

FWIW, does the vessel have any significant "stern walk"?

 

Welcome to the forum Rob,

In addition to the questions from JEH, when you motor ahead, can she hold course without apply any helm?
And if so, is this also true as you go through the 1600-2000 rpm range?

The trialling edges seem odd too...can you can a more clarifying image ?
Since it "looks" like a FB that is offset and parallel to it??
The round tube leading edge...also not great.

As JEH notes, not ideal!

For your application a simple flat plate spade rudder would suffice.

 

Thanks for your reply.

This is all pictures I have.
No stern walk.

 

She is holding the course good at any rpm.

Find one more pictures of the rudder.

If I should rebuild it what is the best option or thing to do?

 

Top and bottom end plates are usually no problem. The 2 supports at the aft edge, either side of the rudder blade, which some bright spark may have thought will help with vectoring thrust, are probably the issue. A few minutes with a grinder.

 

It may not be rudder vibration, but rather a tip clearance issue. Notice the scoured area in the photo. FWIW, rudders of this shape are not unknown. I did a little research and rudders like this are not unknown and typically used for when control is needed at high rudder angles...like a push boat.
Anyway, the distance from the tip of the prop to any structure should be 15-20% of the diameter, so the wheel may be too large for the aperture. This is not a new thing, several vessels I know of have had horrible vibration problems because of this.

See the Boat Mechanical Systems Handbook by Dave Gerr; Chapters 1 and 13.

 

Could the anodes fwd of the prop be causing cavitation?

 

These are either fish tail rudders or Bekker schilling rudders.
They tend not to be issues at any speed, merely providing high thrust at lower speeds.

Agreed. But that tends to apply to the location of the hull, not the skeg, per se.

We have arranged vessels like this in the past, where the prop is very close the skeg (at the lower end), ostensibly for structural reasons, and have not experienced such issues, as described.

I suspect there maybe a structural vibration issue imitated by the engines firing frequency or shaft rpm that sets up the chain of events. As you say it is - in all the boat. That suggests something more fundamental that merely a rudder issue.
Do you know if the vessel had this issue prior to purchase.....did you test her before buying?

 

A yard i worked at had to increase prop size to achieve a contracted speed, and ended up with less than 10% tip clearance. There was no vibration issues, but the prop area had to have extra layers of sound-deadening material. On a later haul out, most of the paint had gone in the local area swept by the blade tips.

 

As above, more than likely a Sympathetic Vibration/Mechanical Resonance issue. Perhaps the frequency in the pulses/change of pressure on the rudder caused by the propeller at the certain rpm coincide with the natural frequency of the rudder.
While many of the examples shown in text discuss this wrt to strings this phenomena applies to structures.

Quoted from a text:
""Every object has a natural frequency at which it will vibrate. These are all examples of a phenomenon called resonance, which is the response of an object that is free to vibrate to a periodic force with the same frequency as the natural frequency of the object. We call this phenomenon mechanical resonance because there is physical contact between the periodic force and the vibrating object. Mechanical resonance must be taken into account when designing bridges, airplane propellers, helicopter rotor blades, turbines for steam generators and jet engines, plumbing systems, and many other types of equipment. A dangerous resonant condition may result if this is not done. For example, in 1940 the Tacoma Narrows suspension bridge in Washington State collapsed when wind caused the bridge to vibrate"" ( There were many reasons involved with the Tacoma bridge failure but the cause has been most often put to
this cause)

An interesting article link:Types of Vibrations On Ships - Machinery Vibrations https://www.marineinsight.com/naval-architecture/types-of-vibrations-on-ships-machinery-vibrations/

 

Old owner says is has always do like this, it was no problem for him, he only drive in the Harbour with the boat.
Today I have bring the boat up on shore and have take some pictures how is looking. In one of the pictures you guys can see the rudder stock and that is shaking when I drive in some rpms.

 

This is the ruder mounting

I have good blacksmith that can build a new rudder. Come with your best ideas for new design

 

Those are better photos, and the two plates either side of the aft edge of the rudder are not fitted as I saw them, so nothing suspect about that. The round bar on the leading edge of the rudder does not fair with the plate, it will leave a lot of vortex behind rather than clean flow. Is the stern tube cutless bearing ok also?